Glass-ceramic composites and glass composites of the type having a matrix of glass or glass ceramic which has been reinforced by filling materials or fibers have become commonly used in a variety of structural applications both as stock pieces and as near net partially formed parts. The properties of these materials which make them attractive for structural applications are their typically low dialectric-constants and loss tangents in the radar frequencies when used as structural components of aircraft for example and high strength over wide temperature ranges. In particular the reasonably high strength to weight of these materials is highly attractive. In addition to their current uses as aircraft, automobile and other structural components, these materials are also being used in an initial or experimental capacity as parts in automotive engines such as gas, turbines diesel and reciprocating engines, and advanced airframe structures including those structural portions which may be impinged by hot exhaust gas. Current and anticipated uses of these materials typically require complex shaped articles for the industrial application desired.
These materials such as glass matrix composites or glass-ceramic matrix composites typically are made up of reinforcing materials such as chopped or continuous fibers of carbon, silicon carbide, aluminum oxide and other conventional fibers or particulate reinforcing such as aluminum oxide, zirconium oxide or silicon carbide or other conventional reinforcing materials imbedded in a matrix of glass or glass-ceramic which can be heat softened at elevated temperatures. Fabrication of parts from glass composites and glass ceramic composites typically consist of two steps, initially a preformed fabrication and followed by a secondary matrix densification. Fiber reinforced preformed articles are typically fabricated by filament winding for example, during which the tows of fiber are infiltrated with a glass slurry. Particular reinforced preformed articles are fabricated typically by slip casting or cold pressing glass powder containing binders and reinforcing material such as chopped fibers or particulate reinforcing. To densify these materials, the preforms are heated above the softening point of the glass in the matrix such that densification occurs through viscous flow of the glass under application of pressure at elevated temperature.
Densification is typically done by hot pressing or hot isostatic pressing. With hot pressing the pressure is applied unidirectionally to a die containing the composite preform. Hot pressing, therefor, is limited to simple geometric shaped articles. Complex shapes can be formed by hot isostatic pressing but the composite preforms must be totally encapsulated by a metal or glass which acts as a pressure bag. The requirement of total encapsulation is a severe limitation for large complex shaped articles. In particular, the cycle time in preparing an article for hot isostatic pressing, completing the pressing cycle and removing the article is quite long and removal can be quite difficult.
Applicant's new technique as described herein permits densification of large and complex shapes using simpler tooling and bagging procedures and with a much lower cycle time. Moreover, applicant's technique permits inexpensive tooling to be used, in particular dies constructed of graphite can be used in applicant's technique without excessive breakage or wear of the dies. If desired conventional dies can also be used.
Applicant's technique uses a sheet of superplastically deformable material to apply the pressure necessary to densify glass and glass ceramic composites as preformed articles. Densification is performed at temperatures above the softening point of the glass matrix. Applicant's process uses conventional preforms of the type known in the art. These materials are cast or laid up in a conventional manner and contain a high degree of void spaces in the intermediate laid up or cast article. In applicant's process densification is performed by placing the article in a suitable mating die. The mating die is placed in a suitable heavy rigid press box, for example made of high temperature steel and a sheet of superplastically formable material is placed over the rigid container to cover the preform and die. A top or cover is placed over the container and traps the edges of the superplastically formable material between the cover and the box itself. The container top and superplastically formable material covering the over lay, the tool and the intermediate part is then placed in a superplastic forming press and trapped between the platens of the press by pressure applied by the press. Heat is applied to the container through the platens by the press heating elements which are a part of the press platens. During this heating step, the interior of the container is evacuated to purge the preform of volatile materials present in the matrix as is known in the art. When the matrix has been thoroughly purged, the temperature of the container and the intermediate part is raised to above the softening point of the glass matrix, for example about 1700.degree. F. At this point, pressure is introduced above the foil of superplastically formable material. It is required that the superplastically formable material be one having a temperature range of superplastic deformation which includes the softening point of the glass matrix used. Typically glasses may be used having a softening point of between about 400.degree. to 500.degree. C. up to as much as 900.degree. to 1000.degree. C. A typical glass matrix material might soften at a temperature of between 1600.degree. to 1700.degree. F. To match this typical material, applicant has found that a foil of Ti-6AL-4V provides a suitable superplastic formable pressure barrier. This material has a superplastic temperature range which includes these conventional glasses. In general applicant's method is suitable for most conventional glass and glass ceramic matrices particularly those having softening points at above 1500.degree. F. Superplastically formable metals, particularly titanium alloys which have a wide superplastic deformation temperature range are known.
When the temperature of the intermediate part and container and superplastic barrier have reached the desired temperature, pressure is introduced above the superplastic barrier to deform the superplastic barrier down around the part and the mating die to partially encapsulate the die and part. In this condition the temperature is maintained above the softening point of the glass and pressure is increased to cause the glass to flow into the void areas of the composite to produce an article having essentially theoretical density.
The advantage of this technique is that the pressure can be applied multi-directionally without requiring the intensive labor typically associated with insuring complete encapsulation of preformed articles with hot isostatic pressing. In addition, degassing of the preformed articles can be easily accomplished in the press. Volatiles can be withdrawn via vents through the container used to house the preformed article and tooling. This permits degassing in situ and handling and use of rigid preforms containing significant amounts of binders. These materials cannot be conveniently densified by vacuum hot pressing and hot isostatic pressing, without additional degassing steps. It is much easier and quicker to perform applicant's process. Applicant's process does not require a complete encapsulation or bagging process and permits a much shorter cycle time and consequently a much lower cost and higher production rate of densified articles. Applicant's process permits easier handling of complex shapes and permits use of inexpensive die materials without excessive die wear and breakage. This procedure permits precisely machined dies and tooling to be used which permits manufacture of dimensionally accurate composite articles which maintain very high tolerances in the composite articles produced. Removal of the part may be simplified for example, by reverse pressure applied between the foil and the composite.
For purposes of this invention glass composites and glass-ceramic composites are considered to be equivalent. Glass-composites have a matrix of amorphous glass. Glass-ceramic composites have an initial matrix of amorphous glass which can be further heat treated to form a crystalline structure.
It is thus an object of applicant's invention to produce a simplified method for multi-directional pressure application to form densified, highly complex shaped glass and glass-ceramic matrix composite.
It is a further object of applicant's invention to produce an inexpensive and more rapidly cycled process of producing densified glass-composite articles.
It is a further object of applicant's invention to produce a process of densifying glass and glass ceramic composites which does not require total encapsulation of the composite article.
It is a further object of applicant's invention to produce a method of densifying glass and glass-ceramic composites which permits in situ degassing of the composites prior to densification.
It is a further object of applicant's invention to produce a method of producing glass and glass ceramic composites which permits dimensionally accurate near net parts and structures to be formed.
These and other objects of applicant's invention will be apparent by reference to the following Description of the Drawings and Description of the Preferred Embodiments.